The present invention relates generally to keyboard electronic musical instruments and, more particularly, to a keyboard electronic musical instrument including a plurality of tone generators substantially less in number than the keyboard keys in response to which the tone generators are operated. More specifically, the invention relates to a tone generator assignment system for use in a keyboard electronic musical instrument of the foregoing type.
Keyboard electronic musical instruments such as electronic organs conventionally include one or more keyboard manuals each comprising a plurality of key switches operable for playing a selected musical arrangement. Traditionally, such instruments were designed by providing a separate, dedicated tone generator for each key switch, each of the tone generators being tuned for producing a tone signal having a frequency corresponding in pitch to the note name of the associated key switch. Needless to say, this design philosophy, although successfully followed for a long period of time, resulted in a relatively complex and costly instrument due at least in part to the multiplicity of tone generators.
More recently, keyboard electronic musical instruments have been proposed which utilize a limited number of tone generators substantially less than the keyboard keys in response to which the tone generators are operated. For example, a 44-note keyboard manual may be played through a tone generation system comprising only eight separate tone generators, it being assumed that no more than eight keys of the manual would normally be played at any given time. While drastically reducing the number of required tone generators, this design approach requires that each tone generator be operable for producing a tone signal whose frequency may be adjusted across the entire range of the keyboard manual. That is, each tone generator must be operable for producing a tone signal whose frequency may be varied for reflecting the pitch corresponding to the note name of each key of the keyboard manual. In addition, a system must be provided for assigning the limited number of tone generators to the keys as they are played. In other words, each time a new key is played one of the tone generators must be assigned for producing a tone signal having a frequency corresponding to the pitch of the note name of the played key. This assignment is preferably made in a manner such that minimal adverse effects are noticed in the musical composition being played. In particular, it is desirable that the newly played key assignment does not prematurely cut-off the tone produced in response to a previously played key. For instance, if a particular tone generator has been assigned for producing a tone signal in response to an initially played key, the tone generator should not be assigned to a newly played key until the envelope or amplitude of the tone signal produced in response to the initially played key has decayed to a sufficiently low level. Therefore, even though the initially played key may have been released, the tone generator assigned thereto should not be re-assigned until the envelope of the tone signal has decayed to a level wherein its abrupt cut-off will not be noticed.
In accordance with the foregoing, known tone generator assignment systems typically include circuitry coupled to the output of each tone generator for sensing the level of the envelope of the tone signals produced thereby. After a key has been released, the tone generator previously assigned thereto is rendered available for re-assignment once the level sensing circuitry determines that the envelope of the tone signal produced thereby has decayed to a sufficiently low reference level. This approach therefore requires the use of relatively complex and costly level sensing circuitry and, in addition, is subject to an undesirable lock-out mode of operation. That is, if all available tone generators are producing tone signals characterized by envelope levels above the reference level, a newly played key will not be assigned and thereby ignored even though one or more of the previously played keys have been released.
It is accordingly a basic object of the present invention to provide an improved tone generator assignment system for a keyboard electronic musical instrument of the type having a plurality of multi-frequency tone generators substantially less in number than the keys in response to which the tone generators are operated.
It is a further object of the invention to provide a tone generator assignment system of the foregoing type which implements a novel tone generator assignment technique in a relatively simple and cost effective manner, the novel tone generator assignment technique providing a musically pleasing effect and being characterized by a reduced likelihood of key lock-out.
It is yet another object of the invention to provide a novel tone generator assignment system which is conveniently implemented in a microprocessor based keyboard electronic musical instrument.